Drilling in the earth is commonly accomplished by using a drill bit having a plurality of rock bit rolling cones ("cutter cones") that are set at angles, through earth formations. The bit essentially crushes the formations through which it drills. The rolling cones rotate on their axes and are, in turn, rotated about the main axis of the drill string. In drilling boreholes for oil and gas wells, blast holes, and raise holes, rock bit rolling cones constantly operate in a highly abrasive environment. This abrasive condition exists during drilling operations even with the use of a medium for cooling, circulating, and flushing the borehole. Such a cooling medium may be either drilling mud, air, or another liquid or gas .
When drilling a hard formation, a bit with tungsten carbide inserts projecting from the body of a rolling cone generally is utilized due to the inserts' relative hardness. However, the carbide inserts are mounted in a relatively soft metal (e.g. steel) that forms the body of the rolling cone. This relatively soft body may be abraded or eroded away when subjected to the high abrasive drilling environment. This abrasion or erosion occurs primarily due to the presence of relatively fine cuttings and chips from the formation that are in the borehole. Additional causes include the direct blasting effect of the drilling fluid utilized in the drilling process, and the rolling or sliding contact of the cone body with the formation. When the material supporting the inserts is substantially eroded or abraded away, the drilling forces either may break the inserts or may force them out of the rolling cone body. As a result, the bit is no longer effective in cutting the formation. Moreover, the inserts that break off from the rolling cone may further damage other inserts, the rolling cones, or other parts of the bit, eventually leading to a catastrophic failure.
Erosion of the rolling cone body usually is most pronounced on the inner and outer edges of the lands of the cone surface. This area is immediately adjacent to the insert and the groove between two rows of inserts. The heaviest wear on the rolling cone surface lands is usually on the inner edges of the outer rows and on the outer edges of the inner rows. When drilling relatively soft but abrasive formations, the bit is able to penetrate at an extremely high rate. This can result in individual cutting inserts penetrating entirely into the abrasive formation causing the formation to come into contact with the cone shell body. When such abrasive contact occurs, the relatively soft cone shell material will wear away at the edges of the surface lands until the interior portion of the insert becomes exposed. The retention ability of the cone body is reduced, thereby ultimately resulting in the potential loss of the insert and reduction of bit life. Because the penetration rate is related to the condition of the bit, the drill bit life and efficiency are of paramount importance in the drilling of boreholes. Accordingly, various methods of hardfacing rock bit cones for erosion or abrasion protection have been attempted. For example, thermal spraying has been used to coat the entire exposed surfaces, including the inserts, of a rolling cone with a hardfacing material. Another method involved placing small, flat-top compacts of hard material in the vulnerable cutter shell areas to prevent cone erosion. Since erosion of groove surface can be the main cause of insert loss due to erosion, methods were developed to apply hardfacing material to both the lands and the grooves of a rolling cone.
It should be noted that inserts are typically retained in a rolling cone by the "hoop" tension generated when the insert is press-fitted into a drilled hole in the rolling cone body. Accordingly, any method to alleviate the erosion of the rolling cone must take into consideration that the "hoop" tension holding the insert must be retained. It has been found undesirable to press the inserts into the cutter before applying hardfacing material. This is because the utilization of heat to adhere the hardfacing material to the surface of the rolling cone relieves the stresses (e.g., "hoop" tension) in the rolling cone. Therefore, it is more desirable to apply hardfacing material to both the lands and grooves of a rolling cone surface for erosion protection before the insert holes are drilled.
For the foregoing reasons, there exists a need for an effective yet economic method of applying hardfacing material to rolling cone surfaces for effective erosion protection. To reduce the cost of manufacturing such rock bits with hardfacing material, it is desirable that the method not be complicated and tedious. Further, the hardfacing material should be applied to the rolling cone surfaces before the insert holes are drilled.